Reliability of duplicate document detection algorithms

ABSTRACT

In a single-signature duplicate document system, a secondary set of attributes is used in addition to a primary set of attributes so as to improve the precision of the system. When the projection of a document onto the primary set of attributes is below a threshold, then a secondary set of attributes is used to supplement the primary lexicon so that the projection is above the threshold.

CLAIM OF PRIORITY

This application claims priority under 35 USC §119(e) to U.S. Provisional Patent Application Ser. No. 60/543,283, filed on Feb. 11, 2004; Ser. No. 60/562,289, filed on Apr. 15, 2004; and Ser. No. 60/580,379, filed on Jun. 18, 2004, all of which are hereby incorporated by reference.

TECHNICAL FIELD

This description relates to duplicate detection and spam filtering.

BACKGROUND

With the advent of the Internet and a decline in computer prices, many people are communicating with one another through computers interconnected by networks. A number of different communication mediums have been developed to facilitate such communications between computer users. One type of prolife communication medium is electronic mail (e-mail).

Unfortunately, because the costs of sending e-mail are relatively low, e-mail recipients are being subjected to mass, unsolicited, commercial e-mailings (colloquially known as e-mail spam or spam e-mails). These are akin to junk mail sent through the postal service. However, because spam e-mail requires neither paper nor postage, the costs incurred by the sender of spam e-mail are quire low when compared to the costs incurred by conventional junk mail senders. Due to this and other factors, e-mail users now receive a significant amount of spam e-mail on daily basis. Spam e-mail impacts both e-mail users and e-mail providers. For e-mail users, spam e-mail can be disruptive, annoying, and time consuming. For an e-mail service provider, spam e-mail represents tangible costs in terms of storage and bandwidth usage. These costs may be substantial when large numbers of spam e-mails are sent.

SUMMARY

In general, when detecting a duplicate document, a primary lexicon of attributes and a secondary lexicon of attributes are generated and unique attributes in a document are determined. An intersection between the unique attributes in the document and the primary lexicon is also determined. When the intersection does not exceed a threshold, attributes from the secondary lexicon that intersect with the unique attributes in the document are added to the intersection to create an augmented intersection that exceeds the threshold. A signature for the document is calculated based on the augmented intersection.

Implementations may include one or more of the following features. For example, the threshold may be a minimum number of attributes or a minimum percentage of the unique attributes in the document. The document may be an e-mail and the unique attributes may include tokens, where the tokens include words in a body of the e-mail. The attributes in the primary lexicon, the attributes in the secondary lexicon, or the unique attributes may include words.

The document may be an unknown document and the signature may be a query signature. The query signature may be compared to known signatures generated from a set of known documents to determine whether the unknown document is a duplicate of one of the set of known documents.

A signature for the document may be calculated based on the intersection when the intersection exceeds the threshold

To generate the primary and second lexicons, a set of documents may be parsed to obtain unique attributes in the set of documents. The normalized inverse document frequency for the unique attributes in the set of documents may be calculated. The primary lexicon may be defined as the unique attributes in the set of documents that have a normalized inverse document frequency within a selected range, while the secondary lexicon may be defined as the unique attributes in the set of documents that have a normalized inverse document frequency outside the selected range, such as, for example, above the selected range. The set of documents may be a set of known spam e-mails or a set of training documents.

To determine the intersection between the unique attributes in the document and the primary lexicon, an intersection may be determined between the unique attributes in the document and the unique attributes in the set of documents to obtain a set of common attributes. A normalized inverse document frequency for the common attributes from the unique attributes in the set of documents may be obtained and the common attributes may be ranked according to the normalized inverse document frequency of the common attributes. The common attributes that have a normalized inverse document frequency within the selected range may be selected to obtain a set of intersection attributes.

To determine whether the intersection exceeds a threshold comprises, a threshold percentage may be multiplied times a number of the unique attributes in the document to obtain a preliminary threshold number. The preliminary threshold number may be compared to a specified threshold number to determine whether the preliminary threshold number exceeds the specified threshold number. A number of attributes in the set of intersection attributes may be compared to the preliminary threshold number when the preliminary threshold number exceeds the specified threshold number. A number of attributes in the set of intersection attributes may be compared to the specified threshold number when the preliminary threshold number does not exceed the specified threshold number.

To add attributes from the secondary lexicon that intersect with the unique attributes in the document to the intersection to create an augmented intersection that exceeds the threshold, an additional number of attributes needed to achieve the threshold may be determined and additional attributes may be selected equal to the additional number of attributes from the set of common attributes that have a normalized inverse document frequency outside of the specified range. The selected attributes may be added to the set of intersection attributes. The selected attributes may be attributes that have a normalized inverse document frequency above the specified range in an increasing order of the normalized document frequency of the additional attributes.

Implementations of the described techniques may include hardware, a method or process, or computer software on a computer-accessible medium.

The details of one or more implementations are set forth in the accompanying drawings and description below. Other features will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of an exemplary networked computing environment that supports e-mail communications and in which spam filtering may be performed.

FIG. 2 is a high-level functional block diagram of an e-mail server program that may execute on an e-mail server to provide large-scale spam filtering.

FIG. 3 is a flowchart of a process that may be employed by an e-mail filter to apply a single-signature-based duplicate detection technique to identify spam e-mails.

FIG. 4 is a flowchart of a process that may be used to implement actions 310 and 320 in FIG. 3 and which employs a secondary lexicon of attributes to increase the reliability or precision of single-signature duplicate detection techniques.

FIGS. 5-8 are flowcharts of exemplary implementations of actions 405, 415, 420 and 425, respectively, in FIG. 4 for an implementation of the process of FIG. 4 based on the I-Match approach.

DETAILED DESCRIPTION

In a single-signature duplicate document system, a secondary set of attributes is used in addition to a primary set of attributes so as to improve the precision of the system. Generally, in single-signature, or single-fingerprint, based duplicate detection systems, two documents are considered identical if their projections onto a lexicon of attributes are the same (where the attributes are typically those that have characteristics useful for identifying a given document). To determine whether the projections are the same, the projections of each document is normally mapped to a single signature, and when two documents' signatures match, they are considered to be duplicate.

Single-signature techniques, however, may provide false positives when the projection of a document onto the lexicon of attributes is small, thereby decreasing the precision of the technique. To mitigate the decrease in precision, when the projection of the document on the lexicon of attributes is below a threshold, then a secondary lexicon of attributes is used to supplement the primary lexicon so that the projection is above the threshold.

Such duplicate detection techniques are described below as applied to e-mail spam filtering. However, the techniques may be used for spam filtering in other messaging media, including both text and non-text media. For example, spam may be sent using instant messaging or short message service (SMS), or may appear on Usenet groups. Similarly, the techniques may be applied, for instance, to filter spam sent in the form of images, sounds, or video when an appropriate set of attributes is selected.

Moreover, the technique described may be applied to duplicate detection problems other than spam filtering. For example, duplicate detection of documents is also useful in Internet search engines and newsfeed applications. For Internet search engines, duplicate detection may be used to eliminate duplicate documents stored as part of the search engine database, or to eliminate duplicate documents from being returned to the user as part of the search results. For newsfeed applications, duplicate detection may be used to insure only one copy of a story is being fed to the user.

More generally, the described techniques may be applied to text or non-text items in a variety of document duplication applications. Therefore, the term “document” should be understood to generally refer to a computer file that contains data for use by applications, such as, for example, a file that contains text, images, sounds, video, other media, or a combination thereof. Accordingly, while described below as being text, the attributes may be text or non-text attributes such that the lexicons of attributes or unique attributes in a document may include both text and non-text attributes, only text attributes, or only non-text attributes.

FIG. 1 illustrates an exemplary networked computing environment 100 that supports e-mail communications and in which spam filtering may be performed. Computer users are distributed geographically and communicate using client systems 110 a and 110 b. Client systems 110 a and 110 b are connected to ISP networks 120 a and 120 b, respectively. While illustrated as ISP networks, networks 120 a or 120 b may be any network, e.g. a corporate network. Clients 110 a and 110 b may be connected to the respective ISP networks 120 a and 120 b through various communication channels such as a modem connected to a telephone line (using, for example, serial line internet protocol (SLIP) or point-to-point protocol (PPP)) or a direct network connection (using, for example, transmission control protocol/internet protocol (TCP/IP)). E-mail or other messaging servers 130 a and 130 b also are connected to ISP networks 120 a and 120 b, respectively. ISP networks 120 a and 120 b are connected to a global network 140 (e.g., the Internet) such that a device on one ISP network can communicate with a device on the other ISP network. For simplicity, only two ISP networks 120 a and 120 b have been illustrated as connected to Internet 140. However, there may be a large number of such ISP networks connected to Internet 140. Likewise, many e-mail servers and many client systems may be connected to each ISP network.

Each of the client systems 110 a and 110 b and e-mail servers 130 a and 130 b may be implemented using, for example, a general-purpose computer capable of responding to and executing instructions in a defined manner, a personal computer, a special-purpose computer, a workstation, a server, a device such as a personal digital assistant (PDA), a component, or other equipment or some combination thereof capable of responding to and executing instructions. Client systems 110 a and 110 b and e-mail servers 130 a and 130 b may receive instructions from, for example, a software application, a program, a piece of code, a device, a computer, a computer system, or a combination thereof, which independently or collectively direct operations. These instructions may take the form of one or more communications programs that facilitate communications between the users of client systems 110 a and 110 b. Such communications programs may include, for example, e-mail programs, IM programs, file transfer protocol (FTP) programs, or voice-over-IP (VoIP) programs. The instructions may be embodied permanently or temporarily in any type of machine, component, equipment, storage medium, or propagated signal that is capable of being delivered to a client system 110 a and 110 b or the e-mail servers 130 a and 130 b.

Each of client systems 110 a and 110 b and e-mail servers 130 a and 130 b includes a communications interface (not shown) used by the communications programs to send communications. The communications may include e-mail, audio data, video data, general binary data, or text data (e.g., data encoded in American Standard Code for Information Interchange (ASCII) format or Unicode).

Examples of ISP networks 120 a and 120 b include Wide Area Networks (WANs), Local Area Networks (LANs), analog or digital wired and wireless telephone networks (e.g., a Public Switched Telephone Network (PSTN), an Integrated Services Digital Network (ISDN), or a Digital Subscriber Line (xDSL)), or any other wired or wireless network including, e.g., a corporate LAN or WAN. Networks 120 a and 120 b may include multiple networks or subnetworks, each of which may include, for example, a wired or wireless data pathway.

Each of e-mail servers 130 a and 130 b may handle e-mail for users connected to ISP network 110 a or 110 b. Each e-mail server may handle e-mail for a single e-mail domain (e.g., aol.com), for a portion of a domain, or for multiple e-mail domains. While not shown, there may be multiple, interconnected e-mail servers working together to provide e-mail service.

An e-mail user, such as a user of client system 110 a or 110 b, typically has one or more e-mail mailboxes on an e-mail system, which may incorporate e-mail server 130 a or 130 b. Each mailbox corresponds to an e-mail address. Each mailbox may have one or more folders in which e-mail is stored. E-mail sent to one of the e-mail user's e-mail addresses is routed to the corresponding e-mail server 130 a or 130 b and placed in the mailbox that corresponds to the e-mail address to which the e-mail was sent. The e-mail user then uses, for example, an e-mail client program executing on client system 110 a or 110 b to retrieve the e-mail from e-mailer server 130 a or 130 b and view the e-mail.

The e-mail client program executing on client systems 110 a and 110 b also may allow one of the users to send e-mail to an e-mail address. For example, the e-mail client program executing on client system 110 a may allow the e-mail user of client system 110 a (the sending suer) to compose an e-mail message and address the message to a recipient address, such as an e-mail address of the user of client system 110 b. When the sender indicates the e-mail is to be sent to the recipient address, the e-mail client program executing on client system 110 a communicates with e-mail server 130 a to handle the sending of the e-mail to the recipient address. For an e-mail addressed to an e-mail user of client system 110 b, for example, e-mail server 130 a sends the e-mail to e-mail server 130 b. E-mail server 130 b receives the e-mail and places it in the mailbox that corresponds to the recipient address. The user of client system 110 b may then retrieve the e-mail from e-mail server 130 b, as described above.

In an e-mail environment such as that shown, a spammer typically uses an e-mail client or server program to send similar spam e-mails to hundreds, if not millions, of e-mail recipients. For example, a spammer may target hundreds of recipient e-mail addresses serviced by e-mail server 130 b on ISP network 120 b. The spammer may maintain the list of targeted recipient addresses as a distribution list. The spammer may use the e-mail program to compose a spam e-mail and instruct the e-mail program to use the distribution list to send the spam e-mail to the recipient addresses. The e-mail is then sent to e-mail server 130 b for delivery to the recipient addresses. Thus, in addition to receiving legitimate e-mails, e-mail sever 130 b also may receive large quantities of spam e-mail, particularly when many hundreds of spammers target e-mail addresses serviced by e-mail server 130 b.

Thus, e-mail systems tend to be used by any given spammer to send large numbers of substantially similar, although non-identical, e-mails. While the content of each spam e-mail contains essentially the same message, the content of each e-mail is normally varied to a degree. For example, mass e-mailings are often personalized by including the recipient user's first/last name or other personal information. Spammers also may add random text to their e-mails so as to foil some spam detection schemes, such as those based on matching exact textual strings in the e-mail. Usually, the core message of the e-mail remains the same, with random or neutral text added to confuse such “exact-match” spam filters. Often the extra text is inserted in such a way that it is not immediately visible to the users (e.g., when the font has the same color as the background). Other randomization strategies of spammers include: appending random character strings to the subject line of the e-mail, changing the order of paragraphs, or randomizing the non-alphanumeric content.

Furthermore, spammers also may change the words used in the e-mail to convey their message. However, because spam e-mails are typically oriented around the same topics (e.g., pornography), the expressiveness of their content is limited. Thus, even though spammers may attempt to randomize the content of their e-mail, the limitation on the expressiveness of their content results in essentially the same e-mail being sent out, even though the e-mails are not exactly identical.

Consequently, duplicate detection systems that identify nearly identical documents may be useful to filter spam e-mails, either when they enter an e-mail system or later on in the e-mail system (e.g., at the recipient's client system). Identification of spam e-mails at the entry point of an e-mail system may be particularly desirable from the e-mail service provider's perspective, as detection at that point may allow the e-mail service provider to prevent the propagation of such e-mails through the system, thereby reducing the waste of computation and storage resources on unsolicited messages.

Referring to FIG. 2, to provide spam filtering by duplicate detection at the entry point of an e-mail system, an e-mail server program 230 may execute on an e-mail system (which may incorporate e-mail server 130 a or 130 b). E-mail server program 230 includes an e-mail filter 232 and a mail handler 234. During operation, the incoming e-mail arriving at e-mail server program 230 is passed to e-mail filter 232. E-mail filter 232 applies duplicate detection techniques to the e-mail to determine whether the e-mail is a spam e-mail or a legitimate e-mail (i.e., not a spam e-mail) and forwards the e-mail to mail handler 234, along with an indication of whether the e-mail is spam or not.

Mail handler 234 then handles the e-mail in a manner that depends on the policies set by the e-mail service provider. For example, mail handler 234 may delete e-mails indicated as spam, while delivering e-mails marked as legitimate to an “inbox” folder of the corresponding e-mail account. Alternatively, e-mail labeled as spam may be delivered to a “spam” folder instead of being deleted.

Referring to FIG. 3, a process 300 may be employed by e-mail filter 232 to apply a single-signature-based duplicate detection technique to identify spam e-mails. In single-signature, or single-fingerprint, based duplicate detection systems, two documents are considered to be the same if their projections onto a lexicon of attributes are the same (where the attributes are typically those that have characteristics useful for identifying a given document). To determine whether the projections are the same, the projection of each document is normally mapped to a single signature, and when two documents' signatures match, they are considered to be duplicates.

For example, in the I-Match approach described by Chowdhury et al. in “Collection Statistics For Fast Duplicate Document Detection,” ACM Transactions on Information Systems, 20(2):171-191, 2002 [hereinafter Chowdhury], two documents are considered to be the same if the projection of the unique words in the documents onto a lexicon of words is the same. To that end, a lexicon of words is developed, where the words chosen are those that have characteristics useful for specifically identifying a given document. More particularly, in I-Match, the lexicon is developed by examining a collection of documents and selecting the words in the collection that have a mid-range inverse document frequency (idf) or normalized inverse document frequency (nidf) (described more fully below).

For a given document, the set of unique words (i.e., each different word) in the document is identified. For example, if the word “cabin” is used in a document multiple times, it is listed once in the set of unique words. The intersection between the set of unique words and the lexicon is obtained (i.e., the words that are in both the lexicon and the set of unique words are identified). This intersection is then mapped to a single hash value using a hash algorithm such as the Secure Hash Algorithm 1 (SHA1) developed by the National Institute of Standards and Technology (described in Chowdhury and in RFC 3174, available at http://www.faqs.org/rfcs/rfc.3174.html). If the hash value matches the hash value of another document, then the two documents are considered to be duplicates of one another.

Using such techniques, after a collection of known spam e-mails has been obtained, the signatures of the known spam e-mails may be calculated and then used to determine if new e-mails are duplicates of the known spam e-mails, and hence, are spam e-mails themselves. Accordingly, in process 300, a set of known spam e-mails is received by e-mail filter 232 (305).

The set of known spam e-mails may be collected in a number of ways. For example, “honeypot” e-mail accounts may be used to collect spam e-mails. A honeypot e-mail account is one set up by an e-mail service provider that does not belong to a legitimate user. The address of the honeypot account is then made available, for example, on websites, usenet groups, or other places where a spammer can obtain the address. Because a honeypot account does not belong to a legitimate user, it should not receive any legitimate e-mails; rather the e-mails received by that account should be spam e-mails. Another manner of collecting spam e-mails may include direct user feedback. To obtain direct user feedback, an e-mail service provider may implement tools or communication channels that allow its users to report spam e-mails that are received. This may be effective because at least some users may be eager to report and/or complain about received spam e-mails. Once a collection of spam e-mails is obtained, human analysts, for example, may review the e-mails to eliminate e-mails that are an exact duplicate of another e-mail in the set. The analysts also may eliminate e-mails that have the exact same message as another e-mail in the set (even though non-message randomization techniques have been applied to make the e-mails non-exact duplicates). The set of known spam e-mails is then provided to e-mail filter 232.

E-mail filter 232 then applies signature-based duplicate detection techniques to obtain a signature (a “spam signature”) for each e-mail in the set of known spam e-mails (310). When the e-mail filter 232 subsequently receives an incoming e-mail (315), e-mail filter 232 applies the duplicate detection techniques to the incoming e-mail to obtain a single signature of the e-mail (a “query signature”) (320). E-mail filter 232 then compares the query signature to the spam signatures to determine if the query signature matches one of the spam signatures (325). If the query signature does not match a spam signature (325), the e-mail filter 232 forwards the incoming e-mail handler 234 with an indication that the incoming e-mail is legitimate. On the other hand, if the query signature does match a spam signature (325), then the incoming e-mail is forwarded to the mail handler 234 with an indication that the e-mail is spam. E-mail handler 234 then handles the incoming e-mail accordingly.

Single-signature techniques, such as I-Match, however, may provide false positives when the projection of a document onto the attributes is small, thereby decreasing the precision of the technique. For example, in the I-Match approach, when the intersection between the set of unique words in a document and the lexicon of words is small, the words used to generate the signature may only be a small portion of the document and, therefore, not very representative of the document. This may result, for instance, in a long document that has the same or nearly the same intersection as a different, smaller document, and, consequently, the I-Match approach may indicate that the longer document is the same as the smaller document, even if this is not the case. In other words, in the I-Match approach, for example, the signature of a document is defined as a hashed representation of the intersection S=(L∩U) (where L is the lexicon and U is the unique words in a document) and this signature becomes unreliable when

$\frac{S}{U}$ becomes too small.

To mitigate such effects in single-signature techniques, a secondary lexicon of attributes (which may be less effective in identifying a given document) may be used to supplement the primary lexicon of attributes when the projection of the document onto the primary lexicon of attributes is below a certain threshold.

Referring to FIG. 4, e-mail filter 232 may implement actions 310 and 320 by using a process 400, which employs a secondary lexicon of attributes to mitigate the above described effects in single-signature techniques so as to increase the reliability of precision of such techniques. The attributes may be, e.g., words, phrases or other clusters of words, HTML tags or other features contained in the body of an e-mail. A word may be defined as a set of alphanumeric characters delimited by whitespace. Additionally, the attributes may be tokenized.

A primary lexicon of tokens L and a secondary lexicon of tokens B are generated (405). The primary lexicon L contains tokens that have characteristics useful for specifically identifying a given document. The secondary lexicon B also has tokens that have characteristics useful for specifically identifying a given document, but the tokens in the secondary lexicon B may be ones that have characteristics that are not as useful as those in the primary lexicon L. For example, the primary lexicon L can be created by analyzing a set of documents to identify by some measure, such as the idf or nidf (described below), those tokens that are most useful in identifying a document and selecting those tokens as the primary lexicon L. The secondary lexicon B then may be created by selecting some or all of the remaining tokens that were in the documents.

For a given e-mail d, e-mail filter 232 then determines the unique tokens U in e-mail d (410). When action 310 is implemented by process 400, e-mail d is one of the e-mails in the set of known spam e-mails. On the other hand, when action 320 is implemented by process 400, e-mail d is the incoming e-mail. To determine the unique tokens U, e-mail filter 232 may, for example, parse the body of e-mail d to tokenize the contents of the body and retain a given token if that token has not been encountered before in the body of e-mail d. In some implementations, e-mail filter 232 may only retain attributes that meet certain criteria (e.g., is at least four characters long or has only one digit) and may apply a common formatting to the attributes (e.g., change all letters to lower case).

Next, e-mail filter 232 determines the intersection between unique tokens U and the primary lexicon L (i.e., e-mail filter 232 determines which tokens in U are also in the primary lexicon L) (415). The intersection is then evaluated to determine if it is above a certain threshold (420). The threshold may be, for example, a minimum number of tokens that is common between the unique tokens U and the primary lexicon L and/or a minimum proportion of common tokens to the tokens contained in U. The value of the threshold is generally a matter of design and may be chosen by the system designers through trial and error, with the threshold selected to obtain a target precision of the system.

If the intersection is below the threshold (425), then tokens from secondary lexicon B that also intersect with the unique tokens U are included in the intersection to achieve an intersection above the threshold. This augmented intersection is then used to calculate the signature for e-mail d (420) by applying an algorithm that generates a single signature to the augmented intersection. On the other hand, if the original intersection is above the threshold, then the original intersection is used to calculate the signature for e-mail d (430) by applying an algorithm that generates a single signature to the original intersection.

If there is another e-mail in the set of known spam e-mails or another incoming e-mail (depending on whether process 400 is implementing action 310 or 320), then process 400 moves to action 410 to find the unique tokens in the other e-mail and the process continues as described above. Otherwise, process 400 is ended and the signature for e-mail d is then stored for future use as a spam signature or compared to a spam signature as a query signature (depending on whether process 400 is implementing action 310 or 320) (440).

FIGS. 5-8 illustrate exemplary implementations of actions 405, 415, 420 and 425, respectively, for an implementation of process 400 based on the I-Match approach.

Referring to FIG. 5, a process 500 is used to generate a primary lexicon L and a secondary lexicon B (action 405). In process 500, a set of documents are used to generate the lexicons. The set of documents may be a training set of documents, related or unrelated to spam e-mails, or the documents may be the set of known spam e-mails, or a combination of both. Even if a large diverse collection of training documents unrelated to spam e-mail is used to generate the lexicons, the lexicons may still be capable of being used to detect duplicates in the spam e-mails. In other words, lexicons generated from a large and diverse set of documents are capable of being used not only to detect duplicates of those documents in the collection, but also documents in different collections. For example, an experiment involved the use of a lexicon created from a corpus of 1.6 million different web pages (which were previously used in the I-Match experiments described in Chowdhury). These lexicons were used in an implementation of process 400 similar to that depicted in FIGS. 5-8, and performed well. This experiment is described in greater detail in U.S. Provisional Patent Application Ser. No. 60/543,283, filed on Feb. 11, 2004.

The ability to use a training collection that can be applied to detect documents in different collections may be useful in the context of spam filtering because the content distribution of spam is constantly changing (i.e., the content or topics of spam are constantly changing). The ability to use a training collection for different documents means that the lexicon(s) do not need to be constantly updated as spam changes, rather, as long as the signatures for the new spam e-mails are calculated and used, e-mail filter 232 should continue to operate well even as the content of spam changes.

Accordingly, in process 500, a set of training documents (related to or unrelated to spam e-mail) is obtained (505), the set of known spam e-mails is used instead (510), or a combination of the two is used to generate the lexicons. To do so, the set of documents used is parsed to obtain the unique tokens in the set of documents (515). Next, the nidf of the unique tokens in the set is calculated (520) according to:

${{NIDF}_{i} = \frac{{IDF}_{i}}{\max\;{IDF}}},{{IDF}_{i} = {\log\frac{N}{n_{i}}}}$ wherein N is the total number of documents in the set, ni is the number of documents in the set that contain token i, and max IDF is the maximum IDF value for the unique tokens. Thus, for example, to calculate the nidf for a particular token, e-mail filter 232 may initially calculate the IDF for all of the unique tokens and then divide the idf for the particular token by the maximum idf value.

Once the nidfs for the unique tokens in the set of documents are calculated, the unique tokens and their respective nidfs are used as a lexicon C. The tokens that have nidfs between a specified range are considered to be the primary lexicon L, while the tokens that have nidfs above the specified range are considered to be the secondary lexicon B. Generally, those tokens with mid-range nidfs are considered to be the ones that are most useful in identifying a particular document. In other words, those tokens that occur very frequently in the set and those that occur infrequently are generally not considered to be as useful in identifying a particular document. Accordingly, the range is generally specified to span those tokens with mid-range nidfs. The exact choice of the specified range is generally in the discretion of the system designer and may involve a degree of trial and error to determine the range that is most useful in a particular system. An exemplary range that may be suitable in some systems is the range of nidfs between 0.2 and 0.3. As suchm the primary lexicon L in this case is considered to be those tokens with nidfs between 0.2 and 0.3, while the secondary lexicon B is considered to be those tokens with nidfs above 0.3.

Referring to FIG. 6, a process 600 is used to determine the intersection between unique tokens U and the primary lexicon L (action 415). Prior to process 600, the unique tokens U in e-mail d are determined (410). A set of common tokens is obtained by determining the intersection between the unique tokens U and the lexicon C (i.e., e-mail filter 232 determines which tokes in U are also in the lexicon C and uses these as a set of common tokens) (605) The common tokens are then ranked according to their nidf (which was obtained from the nidfs contained in lexicon C, i.e., from the nidfs of the unique tokens in the set of documents) (610). The common tokens that have nidfs between the specified range that corresponds to the primary lexicon L are selected to obtain a set of intersection tokens, which are the intersection between the unique tokens U and the primary lexicon L.

Referring to FIG. 7, a process 700 is used to determine if the intersection between the unique tokens U and lexicon L is above the threshold (action 420). In process 700, both a threshold number of tokens and a threshold proportion of unique tokens are used. Specifically, the intersection between the unique tokens U and the primary lexicon L (i.e., the set of intersection tokens) must contain at least a threshold number of tokens (e.g., 5 tokens) or most be at least a threshold proportion or percentage (e.g., 10%) of the unique tokens U, whichever is greater.

The threshold percentage or proportion is multiplied times the number of unique tokens U to determine a preliminary threshold number (705). If the preliminary threshold is greater than or equal to the specified threshold number (710), then a comparison is made between the number of intersection tokens and the preliminary threshold number to determine whether the number of tokens in the intersection set is greater than or equal to the preliminary threshold number (715). If so, then the intersection is above the threshold (730). If not, then the intersection is not above the threshold (720).

If the preliminary threshold is greater than or equal than or equal to the specified threshold number (710), then a comparison is made between the number of intersection tokens and the specified threshold number to determine if the number of intersection tokens is greater than or equal to the specified threshold number (725). If so, then the intersection is above the threshold (730). If not, then the intersection is not above the threshold (720).

The following is an example of process 700 applied to a first set of unique tokens U with 100 tokens, and a second one with 40 tokens when the specified threshold number is 5 and the specified threshold percentage is 10%. For the first set, 10% is multiplied times 100 to obtain a preliminary threshold number of 10 tokens (705). In this case, the preliminary threshold number (i.e., 10) is greater than the specified number (i.e., 5) (710). Thus, for an intersection containing less than 10 tokens (715) the threshold is not met (720), while the threshold is met (730) for an intersection containing 10 or more tokens.

In contrast, for the second set, the preliminary threshold number is less than the specified threshold number (710) (i.e., 10%×40=4, which is less than the specified threshold number 5). Accordingly, the threshold is not met (720) for an intersection that contains less than 5 tokens, but the threshold is met (730) for an intersection with 5 or more tokens.

Referring to FIG. 8, a process 800 is used to add tokens from the secondary lexicon B to the intersection to achieve the threshold (action 425). Based on process 700, the additional number of tokens need to achieve the threshold is determined (805). This can be determined, for example, by taking the difference between number of tokens in the intersection set and the preliminary threshold number or the specified threshold number, depending on the situation that resulted in the threshold not being met. For instance, following the example above, for the first set of unique tokens U that contains 100 tokens, the difference between the number of tokens in the intersection set and the preliminary threshold number is calculated, while the difference between the number of tokens in the intersection set and the specified threshold number is calculated for the second set that contains 40 tokens.

Next, additional tokens from lexicon C are selected (810). The additional tokens selected equal the additional number to achieve the threshold and are selected from the tokens that are considered to be part of lexicon B, i.e., have an nidf above the specified range (e.g., tokens with an nidf greater than 0.3 when a specified range of 0.2 to 0.3 is used). The additional tokens also may be selected in rank order from the lowest nidf in the portion above the specified range to the highest nidf in the portion above the specified range. The tokens may be selected in this order to help insure that the additional tokens have similar nidf characteristics to the ones used from the primary lexicon L and, therefore, are more useful in identifying a particular document. As an example, if two tokens where needed to achieve the threshold, and three tokens have nidfs in range considered to be the secondary lexicon B, then the two tokens with the lowest nidf would be selected.

The additional tokens are then added to the set of intersection tokens to achieve an intersection that is above the threshold (815).

When process 400 is implemented based on an I-Match approach, the set of intersection tokens (which is the intersection) then is used to calculate the signature for the e-mail d (430). To do so, a hash algorithm that maps the set of intersection tokens to a single hash value may be used, where the single hash value is the signature for the e-mail d. For example, the SAH1 algorithm as described and implemented in Chowdhury may be used. The hash value is then stored as a spam signature or used as a query signature, depending on whether process 400 is implementing action 310 or 320.

The above-described techniques also may be used as an integrated system of spam filtering. For example, the duplicate detection technique may be used to quickly identify and eliminate near-duplicates of spam e-mail at the entry point of an e-mail system. The e-mails that are not identified as spam based on duplicate detection may then be classified using, for instance, machine learning techniques. To that end, the e-mails may be provided to a naïve Bayesian classifier or a Support Vector Machine (SVM) based classifier, for instance, which then evaluates the e-mail content and classifies the e-mail spam or non-spam. Thus, in the system of FIG. 2, for example, mail handler 234 may forward the e-mail that is indicated as not being spam to a classifier for classification as spam or non-spam.

The techniques described above are not limited to any particular hardware or software configuration. Rather, they may be implemented using hardware, software, or a combination of both. The methods and processes described may be implemented as computer programs that are executed on programmable computer comprising at least one processor and at least one data storage system. The programs may be implemented in a high-level programming language and may also be implemented in assembly or other lower level languages, if desired.

Any such program will typically be stored on a computer-usable storage medium or device (e.g. CD-ROM RAM, or magnetic disc). When read into the processor of the computer and executed, the instructions of the program cause the programmable computer to carry out the various operations described above.

A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. For example, in some places in the foregoing description an action is described as performed on each e-mail in a group (e.g., a spam signature is obtained for each e-mail in the set of known spam e-mails); however, the performance of the actions on each e-mail is not necessarily required. For instance, with respect to spam signatures, a spam signature may not be obtained for each known spam e-mail because of certain restrictions placed on signatures, such as the requirement that the intersection above or certain threshold can not be obtained for a particular spam e-mail, then the e-mail may be ignored and a signature not generated for it. Similarly, there may be restrictions on the number of unique tokens required. For instance, a spam e-mail may be required to contain a minimum number of unique tokens (e.g., 5) before it is used.

Additionally, in other implementations where the foregoing techniques are applied to spam filtering in other message media or other areas of duplicate detection, the attributes may be other features of the particular items being classified. For instance, the attributes may be n-grams, image features, sound waves, of features extracted from other forms of media.

Accordingly, implementations other than those specifically described are within the scope of the following claims. 

1. A method for use in detecting a duplicate document, the method comprising: generating a primary lexicon of attributes and a secondary lexicon of attributes; determining unique attributes in a document; determining an intersection between the unique attributes in the document and the primary lexicon; determining whether the intersection exceeds a threshold; when the intersection does not exceed the threshold, adding attributes from the secondary lexicon that intersect with the unique attributes in the document to the intersection to create an augmented intersection that exceeds the threshold; and calculating a signature for the document based on the augmented intersection.
 2. The method of claim 1 further comprising calculating a signature for the document based on the intersection when the intersection exceeds the threshold.
 3. The method of claim 1 wherein generating a primary lexicon of attributes and a secondary lexicon of attributes comprises: parsing a set of documents to obtain unique attributes in the set of documents; calculating the normalized inverse document frequency for the unique attributes in the set of documents; defining the primary lexicon as the unique attributes in the range of documents that have a normalized inverse document frequency within a selected range; and defining the secondary lexicon as the unique attributes in the set of documents that have a normalized inverse document frequency outside the selected range.
 4. The method of claim 3 wherein defining the secondary lexicon comprises defining a secondary lexicon as the unique attributes in the set of documents that have a normalized inverse document frequency above the selected range.
 5. The method of claim 3 wherein the set of documents is a set of known spam e-mails.
 6. The method of claim 3 wherein the set of documents is a set of training documents.
 7. The method of claim 3 wherein determining an intersection between the unique attributes in the document and the primary lexicon comprises: determining an intersection between the unique attributes in the document and the unique attributes in the set of documents to obtain a set of common attributes; obtaining a normalized inverse document frequency for the common attributes from the unique attributes in the set of documents; ranking the common attributes according to the normalized inverse document frequency of the common attributes; and selecting the common attributes that have a normalized inverse document frequency within the selected range to obtain a set of intersection attributes.
 8. The method of claim 7 wherein determining whether the intersection exceeds a threshold comprises: multiplying a threshold percentage times a number of the unique attributes in the document to obtain a preliminary threshold number; comparing the preliminary threshold number to a specified threshold number to determine whether the preliminary threshold number exceeds the specified threshold number; comparing a number of attributes in the set of intersection attributes to the preliminary threshold number when the preliminary threshold number exceeds the specified threshold number; and comparing a number of attributes in the set of intersection attributes to the specified threshold number when the preliminary threshold number does not exceed the specified threshold number.
 9. The method of claim 7 wherein adding attributes from the secondary lexicon that intersect with the unique attributes in the document to the intersection to create an augmented intersection that exceeds the threshold comprises: determining an additional number of attributes needed to achieve the threshold; selecting additional attributes equal to the additional number of attributes from the set of common attributes that have a normalized inverse document frequency outside of the specified range; and adding the additional attributes selected to the set of intersection attributes.
 10. The method of claim 9 wherein selecting additional attributes equal to the additional number of attributes from the set of common attributes that have a normalized inverse document frequency outside of the specified range comprises selecting additional attributes from the set of common attributes that have a normalized inverse document frequency above the specified range in an increasing order of the normalized document frequency of the additional attributes.
 11. The method of claim 1 wherein the threshold is a minimum number of attributes.
 12. The method of claim 1 wherein the threshold is a minimum percentage of the unique attributes in the document.
 13. The method of claim 1 wherein the document is an e-mail.
 14. The method of claim 13 wherein the wherein the unique attributes comprise tokens.
 15. The method of claim 14 wherein the tokens comprise words in a body of the e-mail.
 16. The method of claim 1 wherein the attributes in the primary lexicon, the attributes in the secondary lexicon, and the unique attributes comprise words.
 17. The method of claim 1 wherein the document is an unknown document and the signature is a query signature, the method further comprising: comparing the query signature to known signatures generated from a set of known documents to determine whether the unknown document is a duplicate of one of the set of known documents.
 18. The method of claim 1 further comprising determining an intersection between the unique attributes in the document and the secondary lexicon.
 19. A comparable-user medium having a computer program embodied thereon for use in detecting a duplicate document, the comparator program comprising instructions for causing a computer to perform the following operations: generate a primary lexicon of attributes and a secondary lexicon of attributes; determine unique attributes in a document; determine an intersection between the unique attributes in the document and the primary lexicon; determining whether the intersection exceeds a threshold; when the intersection does not exceed the threshold, add attributes from the secondary lexicon that intersect with the unique attributes in the document to the intersection to create an augmented intersection that exceeds the threshold; and calculate a signature for the document based on the augmented intersection.
 20. The medium of claim 19 wherein, to generate a primary lexicon of attributes and a secondary lexicon of attributes, the computer program further comprises instructions for causing a computer to: parse a set of documents to obtain unique attributes in the set of documents; calculate the normalized inverse document frequency for the unique attributes in the set of documents; define the primary lexicon as the unique attributes in the set of documents that have a normalized inverse document frequency within a selected range; and define the secondary lexicon as the unique attributes in the set of documents that have a normalized inverse document frequency outside the selected range.
 21. The medium of claim 20 wherein, to define the secondary lexicon, the computer program further comprises instructions for causing a computer to define the secondary lexicon as the unique attributes in the set of documents that have a normalized inverse document frequency above the selected range.
 22. The medium of claim 20 wherein the set of documents is a set of known spam e-mails.
 23. The medium of claim 20 wherein, to determine an intersection between the unique attributes in the document and the primary lexicon, the computer program further comprises instructions for causing a computer to: determine an intersection between the unique attributes in the document and the unique attributes in the set of documents to obtain a set of common attributes; obtain a normalized inverse document frequency for the common attributes from the unique attributes in the set of documents; rank the common attributes according to the normalized inverse document frequency of the common attributes; and select the common attributes that have a normalized inverse downward frequency within the selected range to obtain a set of intersection attributes.
 24. The medium of claim 23 wherein, to determine whether the intersection exceeds a threshold, the computer program further comprises instructions for causing a computer to: multiply a threshold percentage times a number of unique attributes in the document to obtain a preliminary threshold number; compare the preliminary threshold number to a specific threshold number to determine whether the preliminary threshold number exceeds the specified threshold number; compare a number of attributes in the set of intersection attributes to the preliminary threshold number when the preliminary threshold number exceeds the specified threshold number; and compare a number of attributes in the set of intersection attributes to the specified threshold number when the preliminary threshold number does not exceed the specified threshold number.
 25. The medium of claim 23 wherein, to add attributes from the secondary lexicon that intersect with the unique attributes in the document to the intersection to create an augmented intersection that exceeds the threshold, the computer program further comprises instructions for causing a computer to: determine an additional number of attributes needed to achieve the threshold; select additional attributes equal to the additional number of attributes from the set of common attributes that have a normalized inverse document frequency outside of the specified range; and add the additional attributes selected to the set of intersection attributes.
 26. The medium of claim 25 wherein, to select additional attributes equal to the additional number of attributes from the set of common attributes that have a normalized inverse document frequency outside of the specified range, the computer program further comprises instructions for causing a computer to select additional attributes from the set of common attributes that have a normalized inverse document frequency above the specified range in an increasing order to the normalized document frequency of the additional attributes.
 27. The medium of claim 19 wherein the document is an e-mail.
 28. The medium of claim 19 wherein the attributes in the primary lexicon, the attributes in the secondary lexicon, and the unique attributes comprise words.
 29. The medium of claim 19 wherein the document is an unknown document and the signature is a query signature, the computer program further comprising instructions for causing a computer to compare the query signature to known signatures generated from a set of known documents to determine whether the unknown document is a duplicate of one of the set of known documents.
 30. The medium of claim 19 wherein the computer program further comprises instructions for causing a computer to determine an intersection between the unique attributes in the document and the secondary lexicon.
 31. An apparatus for use in detecting a duplicate document, the apparatus comprising: means for generating a primary lexicon of attributes and a secondary lexicon of attributes; means for determining unique attributes in a document; means for determining an intersection between the unique attributes in the document and the primary lexicon; means for determining whether the intersection exceeds a threshold; when the intersection does not exceed the threshold, means for adding attributes from the secondary lexicon that intersect with the unique attributes in the document to the intersection to create an augmented intersection that exceeds the threshold; and means for calculating a signature for the document based on the augmented intersection. 